Patent GB-1410633-A (GB)
Title: Apparatus for measuring the resistance of a variable resistance  element located in a rotating body
IPC: G01K7/20
Filed: 1972-09-07
Abstract: 1410633 Bridge circuits: measuring temperature TORAY ENG CO Ltd 7 Sept 1972 41600/72 Headings G1N and G1U [Also in Division G3] A resistance element 3, Fig. 1, responsive to the temperature of a heat roller 1 in which it is embedded, is coupled into an A.C. bridge for detecting departures of the temperature from a pre-set value, by a rotating transformer 12 having a rotating primary winding 7 on the shaft 9 of the roller and a stationary secondary winding 8. The secondary winding may also have a resistive ambient-temperature compensating winding 14, Fig. 2, bifilar wound with it, and both windings may be brought out to terminals 15, 16, 17, as shown, via a junction box 11 which may include the components 12 and 13. In a first bridge circuit 30a, Fig. 5, to which the transducer unit, Figs. 1 and 2, may be coupled via a cable 26 having inter-conductor capacitances 18, 19, two arms are constituted by the tapped winding of a supply transformer 24, the third arm by set-point adjusting resistor 21, the capacitor 12 and the rotating transformer 8, the fourth arm by a resistor 22 and the resistors 13, 14, and an output signal, for controlling a heater 33, is produced by rectifying 35, 35a, the signal e 1  across the bridge diagonal and the signal e2 across the lower portion of the supply transformer, and subtracting the  resulting D.C. signals in a differential amplifier 36. In use the capacitor 12 is tuned to resonate at the supply frequency with the rotating transformer 8 so that, as a consequence, the reactive component of impedance presented to the bridge by the two left-hand arms is minimized and the sensitivity of the bridge to departures of the temperature of the resistive element 3 from the set temperature is both maximized and rendered substantially independent of small variations in the inductance 8 of the rotating transformer. The theory of the bridge is outlined Figs. 6A, 6B, 6C (not shown). In a second bridge circuit, Fig. 4, the reactive component presented by the two left-hand arms is brought to zero by using two additional capacitors 18a and 20, so that providing the reactance of the inter-conductor capacitance 19 is small relative to the resistors 13, 14, in parallel across it, the bridge is effectively an A.C. energized resistive bridge and the diagonal voltage e 1  can be used directly, via an amplifier 31 and phase sensitive rectifier 34 (since the error may be of either sign), as an error voltage to control the heater 33. Simplified versions of the bridges of Figs. 4 and 5 obtained by omitting the ambient temperature compensating resistor 14 are described Figs. 9A, 9B, 9C and 8, respectively, (not shown), and it is also shown Fig. 7 (not shown), how the second of these simplified bridges may be combined with two temperature sensors (3-1, 3-2) so as to maintain two rollers at equal temperatures. Suitable arrangements for effecting the comparison of voltages required with the bridge of Fig. 5 so as to avoid the use of D.C. amplifiers with their inevitable drift are also outlined Figs. 11 and 12 (not shown).